Welcome! And thank you for taking in an older cat.
Weight loss in older cats can be caused by a number of problems -- thyroid, diabetes, CRF, IBS, etc. I know you said you had bloodwork done, but vets may or may not run a thyroid values, which is where I'd start looking for answers. The second place I'd look is the people that you adopted him from and find out as much history as I could, especially about what he ate and this constipation issue.
In terms of getting him to eat more, yes, constipation can decrease eating and cause weight loss. You don't mention how constipated he is (details please), but there are two easy fixes. First, try mixing 1 tsp of canned pumpkin (not pumpkin pie filling -- straight pumpkin) into his wet food. Pumpkin is excellent for adding fiber and stopping constipation. Second, you can use a hairball remedy 2-3 times a week. This is a second choice because most are mineral oil based which interfers with some nutrient absorption.
Another thing to do is try to increase the appeal of his food. You can top his wet food with various things such as Gerbers babyfood (the all meat varieties -- no vegies or onions), the juice from canned tuna/salmon/mackeral, bonita flakes, even powdered cat treats. Often a stronger smell will stimulate more eating.
You can also increase the calories of what food he is eating. Don't take this wrong, but Fancy Feast isn't the best food for calories per tablespoon. Go to a pet store and find some kitten food. Kitten food is higher calorie without a lot of filler. Alternatively, ask your vet for some Science Diet A/D or Iams Max Calorie -- both are great for helping add weight/maintain weight on sick or recovering cats. Another alternative is a product called NutriCal. It comes in a tube, like toothpaste, and is a high calorie dietary supplement. Put some on your finger and hopefully he'll lick it off.
Good luck and thanks again for giving him a new home!